Lecture 11: Consistency Models

1. Lecture 11: Consistency Models • Topics: sequential consistency, hw and hw/sw optimizations

2. Coherence Vs. Consistency • Recall that coherence guarantees (i) that a write will eventually be seen by other processors, and (ii) write serialization (all processors see writes to the same location in the same order) • The consistency model defines the ordering of writes and reads to different memory locations – the hardware guarantees a certain consistency model and the programmer attempts to write correct programs with those assumptions

3. Example Programs Initially, A = B = 0 P1 P2 A = 1 B = 1 if (B == 0) if (A == 0) critical section critical section Initially, A = B = 0 P1 P2 P3 A = 1 if (A == 1) B = 1 if (B == 1) register = A P1 P2 Data = 2000 while (Head == 0) Head = 1 { } … = Data

4. Sequential Consistency P1 P2 Instr-a Instr-A Instr-b Instr-B Instr-c Instr-C Instr-d Instr-D … … We assume: • Within a program, program order is preserved • Each instruction executes atomically • Instructions from different threads can be interleaved arbitrarily Valid executions: abAcBCDdeE… or ABCDEFabGc… or abcAdBe… or aAbBcCdDeE… or …..

5. Sequential Consistency • Programmers assume SC; makes it much easier to reason about program behavior • Hardware innovations can disrupt the SC model • For example, if we assume write buffers, or out-of-order execution, or if we drop ACKS in the coherence protocol, the previous programs yield unexpected outputs

6. Consistency Example - I • Consider a multiprocessor with bus-based snooping cache coherence and a write buffer between CPU and cache Initially A = B = 0 P1 P2 A  1 B  1 … … if (B == 0) if (A == 0) Crit.Section Crit.Section The programmer expected the above code to implement a lock – because of write buffering, both processors can enter the critical section The consistency model lets the programmer know what assumptions they can make about the hardware’s reordering capabilities

7. Consistency Example - 2 P1 P2 Data = 2000 while (Head == 0) { } Head = 1 … = Data Sequential consistency requires program order -- the write to Data has to complete before the write to Head can begin -- the read of Head has to complete before the read of Data can begin

8. Consistency Example - 3 Initially, A = B = 0 P1 P2 P3 A = 1 if (A == 1) B = 1 if (B == 1) register = A Sequential consistency can be had if a process makes sure that everyone has seen an update before that value is read – else, write atomicity is violated

9. Sequential Consistency • A multiprocessor is sequentially consistent if the result of the execution is achieveable by maintaining program order within a processor and interleaving accesses by different processors in an arbitrary fashion • The multiprocessors in the previous examples are not sequentially consistent • Can implement sequential consistency by requiring the following: program order, write serialization, everyone has seen an update before a value is read – very intuitive for the programmer, but extremely slow

10. HW Performance Optimizations • Program order is a major constraint – the following try to get around this constraint without violating seq. consistency • if a write has been stalled, prefetch the block in exclusive state to reduce traffic when the write happens • allow out-of-order reads with the facility to rollback if the ROB detects a violation (detected by re-executing the read later)

11. Relaxed Consistency Models (HW/SW) • We want an intuitive programming model (such as sequential consistency) and we want high performance • We care about data races and re-ordering constraints for some parts of the program and not for others – hence, we will relax some of the constraints for sequential consistency for most of the program, but enforce them for specific portions of the code • Fence instructions are special instructions that require all previous memory accesses to complete before proceeding (sequential consistency)

12. Fences P1 P2 { { Region of code Region of code with no races with no races } } Fence Fence Acquire_lock Acquire_lock Fence Fence { { Racy code Racy code } } Fence Fence Release_lock Release_lock Fence Fence

13. Potential Relaxations • Program Order: (all refer to different memory locations) • Write to Read program order • Write to Write program order • Read to Read and Read to Write program orders • Write Atomicity: (refers to same memory location) • Read others’ write early • Write Atomicity and Program Order: • Read own write early

14. Relaxations • IBM 370: a read can complete before an earlier write to a different address, but a read cannot return the value of a write unless all processors have seen the write • SPARC V8 Total Store Ordering (TSO): a read can complete before an earlier write to a different address, but a read cannot return the value of a write by another processor unless all processors have seen the write (it returns the value of own write before others see it) • Processor Consistency (PC): a read can complete before an earlier write (by any processor to any memory location) has been made visible to all

15. Performance Comparison • Taken from Gharachorloo, Gupta, Hennessy, ASPLOS’91 • Studies three benchmark programs and three different architectures: • MP3D: 3-D particle simulator • LU: LU-decomposition for dense matrices • PTHOR: logic simulator • LFC: aggressive; lockup-free caches, write buffer with bypassing • RDBYP: only write buffer with bypassing • BASIC: no write buffer, no lockup-free caches

16. Performance Comparison

17. Summary • Sequential Consistency restricts performance (even more when memory and network latencies increase relative to processor speeds) • Relaxed memory models relax different combinations of the five constraints for SC • Most commercial systems are not sequentially consistent and rely on the programmer to insert appropriate fence instructions to provide the illusion of SC

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